If the device is in debug interface mode, System OFF will be emulated to secure that all required resources needed for debugging are available during System OFF.
See Debug and trace chapter for more information. Required resources needed for debugging include the following key components: Debug and trace, CLOCK — Clock control, POWER — Power control, NVMC — Non-volatile memory controller, CPU, flash, and RAM. Since the CPU is kept on in emulated System OFF mode, it is required to add an infinite loop directly after entering System OFF, to prevent the CPU from executing code that normally should not be executed.